Gliederung

Question: Strength-Duration (SD) curves are widely used to assess optimal stimulation intensity in cardiac pacing and defibrillation, but their potential as a tool to describe electrophysiological tissue properties is still poorly investigated. This study aims to define myocardial excitability in rat hearts in vivo under various experimental conditions (health/aging/diabetes), by means of SD curves recorded during high spatial resolution epicardial mapping.

Method Used: Unipolar potentials were measured from the anterior ventricular epicardium of rat hearts in situ (Wistar+Wild Type, M+F, 6- to 24-month-old) by means of 1 mm resolution electrode arrays. Threshold intensities were determined for cathodal unipolar stimuli of increasing duration (0,01 to 5 ms) delivered to the epicardium at several array electrodes. SD curves were computed after the modified Weiss-Lapicque model to quantify Rheobase (Irh) and Chronaxie (c).

Results: Cardiac excitability identified by Irh and c is subject to great individual variability and is spatially heterogeneous in the ventricle, seemingly independent from the presence of pathological alterations. Besides, experimental data show remarkable deviations from the Lapicque model at short stimulus durations (0.01 to 0.1 ms). However, excitability is higher in senescent hearts compared to control, as shown by decrease of Irh and leftward curve shift. No differences were detected in diabetic animals.

Conclusion: SD curves have proven useful to study excitability in vivo, and the electrophysiological outcome of common pathological conditions such as age-related myocardial fibrosis or hypertrophy. Increased excitability may help explain enhanced susceptibility to arrhythmogenic stimuli in aging. However, improved SD models should be devised as the cable-based Lapicque equation fails to fully account for the three-dimensional, anisotropic, non-uniform structure of the myocardium.